1. Field of the Invention
The present invention relates to an apparatus for fabricating a hard carbon coating to be prepared as a surface protective film for a magnetic tape, an optical magnetic disc or the like, and a method of manufacturing such a hard carbon coating. Further, the present invention relates to an apparatus for manufacturing a magnetic disc medium having high durability and high recording density and excellent in productivity on a polymeric substrate material, and more particularly to an apparatus for manufacturing a protective film having functions of wear resistance and lubricating ability, which is industrially applied to an image equipment, an information equipment field and the like.
2. Description of the Related Art
Hitherto, there has been known a technique by which a diamond-shaped carbon coating is formed. The diamond-shaped carbon coating has a diamond structure and is also called a diamond-like carbon (DLC) film. Hereinafter, this carbon coating is referred to as xe2x80x9chard carbon coatingxe2x80x9d.
The hard carbon coating is coated on a surface of resin or a polymeric film and can be used as a wear-resistant layer or a protective layer. When manufacturing the hard carbon coating, a film forming apparatus such as a plasma CVD apparatus shown in FIG. 2 is used. In the film forming apparatus shown in FIG. 2, a pair of electrodes 112 and 114 are disposed within a vacuum vessel 111. One electrode 112 is connected to a high-frequency power source 115 (generally, 13.56 MHz) whereas the other electrode 114 is grounded. An object or a substrate 113 on which a film is to be formed is disposed on the electrode 112 side to which a high-frequency power (voltage) is supplied. Also, although not shown, a supply system, an exhaust system for a reactive gas, and a matching device for supplying voltage are provided.
In the plasma CVD apparatus shown in FIG. 2, electrons are charged onto the substrate 113 and the electrode 112 (electrode opposed to the grounded electrode 114) connected to the high-frequency power source 115. Therefore, H+-ions and H-radicals contributing to heightened quality of the film by action of self-bias collide with the object 113, thereby to prepare the carbon coating having the diamond structure.
The hard carbon coating thus prepared can be used as the protective film of the magnetic recording medium such as a magnetic tape or an optical magnetic disc, etc. Since these magnetic recording media are made of magnetic material, it is necessary to protect the media from being mixed with a foreign matter or being damaged. For example, a DC bias is applied in addition to high-frequency discharge, whereby a carbon coating in which pin-holes of 102 to 105 per mm2 are formed is prepared on a surface of the magnetic recording medium.
However, according to the experiments of the inventors, it has been ascertained that the hard carbon coating having the pin-holes therein lacks long-term reliability as a protective film because moisture is infiltrated into the pin-holes. Also, it has been ascertained that improvements in hardness and adhesion of the hard carbon coating and the prevention of generation of the pin-holes are not always performed together.
Recently, there is a tendency to heighten the density of the magnetic recording medium. As a conventional magnetic recording medium, there has been known a coating type in which xcex3-Fe2O3 powders, CrO powders, pure iron powders or the like which are used for an audio or video tape material are coated on a polymeric substrate material together with an abrasive material and a binder. Further, a high performance magnetic recording medium on which a metal magnetic material has been vapor-deposited is used.
Furthermore, there has been known a technique by which a coating mainly containing carbon (also called a carbon film, a DLC or a hard carbon film) is formed on a surface of these magnetic recording medium, thereby to obtain a surface protection, a wear resistance or a lubricating ability. In general, the coating mainly containing carbon is fabricated by the CVD method typical of the plasma CVD method.
In the typical plasma CVD method, a substrate is located on a high-frequency voltage supply electrode (cathode) side, and a self-bias formed in the vicinity of the cathode allows a high-hard film to be fabricated. In general, a carbon film with a high hardness cannot be fabricated on a grounded electrode (anode) side.
When a coating mainly containing carbon is formed using the plasma CVD method of the parallel plate type, an organic resin substrate constituting an object of the magnetic recording medium must be located on a cathode electrode side. The magnetic recording medium for high-density recording is generally obtained by vapor-depositing the metal magnetic material. Therefore, if such an object is in contact with the cathode electrode, the object constitutes a part of the electrode, and therefore a high-frequency electric field is leaked as a result of which discharge occurs in an undesired region. There is a high possibility that the organic resin film constituting the object is damaged by such discharge, causing a problem on the stability and reliability of production.
Furthermore, fabricating the hard carbon film which is a protective film at the same time as the roll to roll type magnetic layer fabricating process is impossible because the carbon film forming speed is low.
An object of the present invention is to provide a method of forming a novel hard carbon coating.
Another object of the invention is to provide a method of fabricating a hard carbon coating which is fine, has high hardness and adhesion and reduces the number of pin-holes formed therein as a protective film of a magnetic recording medium.
Still another object of the invention is to provide an apparatus capable of stably producing a hard carbon film on a surface of a magnetic recording medium including an electrically conductive metal magnetic layer with high reliability, that is, to provide an apparatus capable of fabricating a carbon film having a sufficient wear-resistance and lubricating ability under the condition where it is in contact with an anode which constitutes a grounded electrode.
Yet still another object of the invention is to provide an apparatus capable of forming a film at a high speed to the degree of being capable of fabricating a hard carbon film which is a protective film at the same time as a process of fabricating a magnetic layer.
Yet still another object of the invention is to provide an apparatus capable of restraining generation of a flake caused by a stain of an electrode, by forming a film at a high speed.
In the present invention, the hard carbon coating is fabricated while an object is subjected to ultrasonic vibrations. In particular, in the plasma CVD apparatus constituted by a parallel plate in which a high-frequency power supply is connected to one electrode on a side where the substrate on which the hard carbon coating is to be formed is located, and the other electrode is grounded, the carbon coating is fabricated under the condition where the ultrasonic vibrations is propagated to the substrate.
Further, in the present invention, a magnetic recording medium such as a band-shaped film object (for example, a taped film) or the like is used for an object, this object travels while it is subjected to ultrasonic vibrations, and the hard carbon coating is fabricated an the surface of the object (substrate) as a protective film.
By subjecting the object to ultrasonic vibrations, cluster-shaped carbon of small particles or carbon molecules can be deposited on the surface of the object, and the carbon coating formed can be fine and uniform in quality. This is because the object is ultrasonic-vibrated, whereby large carbon molecules is repelled from the substrate which is vibrating and molecules having a specified size or less are liable to be deposited on the surface of the object.
Further, a film shaped object is used for an object, and the object vibrates in a direction of an inter-electrode, thereby to realize a state where bias is applied in a pulse mode or a high-frequency mode.
Still further, the number of pin-holes per 1 mm2 in the hard carbon coating is 30 or less. The thickness of the hard carbon coating is 50 to 2000 xc3x85, preferably 100 to 500 xc3x85.
If at lease one kind of element selected from Si, B, N, P and F is contained by 20 atoms % or less in a hard carbon coating used as this protective film, adhesion of the coating is improved to provide electrical conductivity. For example, Si and P are contained in the hard carbon coating, thereby to obtain a protective film having high conductivity and making it difficult to charge with static electricity.
When using a slender film-shaped object such as a magnetic tape, ultrasonic vibrations are given to the object, thereby being capable of preventing the substrate from being contracted. Also, particles (a powder like raw material which does not come to a film) attached to the surface of the substrate can be removed by ultrasonic vibrations.
Further, ultrasonic vibrations are given to the substrate in the direction of the inter-electrode arranged in parallel, thereby to realize a state where AC bias voltage is applied between electrodes. This action is particularly effective in the case of using a film-shaped substrate such as a magnetic tape which is flexible and has a large amplitude.
According to the present invention, in an apparatus for fabricating a coating in which a first electrode to which high-frequency electric field is applied and a grounded second electrode are arranged so as to be opposed to each other, high-frequency electric field is applied to generate plasma between the first and second electrodes, and raw gas introduced in the plasma is activated to fabricate a coating, an interval between the first and second electrodes is 6 mm or less, pressure between the electrodes is 15 to 100 Torr. That is, when the electrode interval is 6 mm or less and pressure therebetween is 15 to 100 Torr, even though the substrate is in contact with the second electrode, the carbon film with high hardness can be fabricated. This is ascertained by the experimental view of the inventors.
Prior to the above-mentioned view, the inventors have observed the physical property of a plasma within a high pressure range (5 Torr to 760 Torr) remarkably higher than a pressure range (10 mTorr to 1 Torr) generally selected by the plasma CVD. The reason why attention is paid to the pressure range higher than what is generally expected is that the speed of film forming by the usual plasma CVD is desirable to improve excessively.
Considering film forming processes in the plasma CVD (generation of a radical, transport of the radical onto the surface of the substrate, and reaction of the radical on the surface), if (1) a radical density forming a precursor of a film, and (2) the transport efficiency of the radical onto the surface of the substrate can be improved, then a film forming speed can be improved. In the case of the plasma CVD, since the radical occurs in the whole plasma space, it can be presumed that the generation of the radical influences the film forming speed greater than the transport of the radical. The radical density can be increased by elevating a reaction pressure, and it can be expected that the film formation under high pressure results in a high-speed film formation.
In the film forming process, (3) the reaction of the radical on the surface of the film (suppression of surface desorption) is considered other than the above-mentioned cases (1) and (2). However, in the case of a low-temperature process such as the plasma CVD, the surface reaction rate is not determined, but the reaction process of the radical on the surface of the film to the film forming speed does not contributed thereto. In the case of forming the hard carbon film, the ion action on the surface greatly influences the film quality. That is, bombardment of ions positively acts while the hard carbon film is formed, whereby strong coupling in the film remains whereas weak coupling is cut off. Therefore, in general, the object is disposed on the cathode side, and a film is formed using self-bias.
Even though the increase in the radical density can be realized by pressure increase when forming a film, there is no sense in the case of greatly changing the physical property of plasma which is a premise of the radical generation by pressure rise. Accordingly, the inventors have observed plasma in high pressure range (5 Torr to 760 Torr), as mentioned above.
First, requirements for generating plasma in the high pressure range (5 Torr to 760 Torr) must be set. The reason that low-pressure glow discharge is conventionally generated in a pressure range of 10 mTorr to 1 Torr is because discharge is most liable to occur in this pressure range, that is, discharge is stabilized. The number of times when particles existing between the parallel plate electrodes having an electrode interval d (in the case of the usual low-pressure glow discharge, d=several tens mm) collide with electrons (it is assumed that electrons are accelerated by electric field between electrodes so as to fly from one electrode to the other electrode) is proportional to its atmospheric pressure. That is, the number of times is inverse proportional to a mean free stroke. Therefore, when the pressure is low and the number of times of collision is reduced, because electrons have sufficient energy, ionization of the particles is generated by that collision. However, due to low pressure, the particles per se are reduced thereby not forming plasma. On the other hand, when pressure is high, the number of collision times of electrons is increased whereby electrons cannot have sufficient energy till the succeeding collision, as a result of which the particles cannot be ionized even though they collide with the electrons. This is known as Paschen""s law, and discharge start voltage V becomes a function of product (pd product) of pressure p and an electrode interval d, so that a minimum discharge start voltage Vmin exists when the pd product has a certain value. That is, when plasma is to be generated in a high-pressure range, it is required that sufficient electric field is given to electrons in order to ionize particles for a short free stroke.
To cope with this, the electrode interval d is narrowed and voltage applied between electrodes is heightened. There is a limit in an effect resulting from heightening the voltage applied between the electrodes. That is, in the case of glow discharge, an electric field distribution within plasma is not uniform so that an electric field is largest on a sheath portion formed in vicinity of the electrodes. Then, an electric field is developed on a positive column portion connected to the sheath portion. The length of the sheath portion is as long as a Debye length inherent to plasma, and the electric field of the positive column portion occupying most of space is not so large. Therefore, even though large voltage is applied between electrodes, a substantial increase of the electric field on the positive column portion occupying most of space cannot be much expected.
Because an increase in voltage between the electrodes is applied to the sheath portion, the ionization in this range is facilitated. When the electric field on the sheath portion exceeds a predetermined strength, accelerated electrons collides with the surface of the electrode, thereby to cause thermionic emission. A mechanism of discharging electronics from the electrode in a glow discharge state is field emission and secondary electron emission. However, when the thermionic emission is generated, the electric field consumed with electron emission from the electrode does not almost exist, and the electric field of that amount occurs on the sheath portion. In this case, electrons on the sheath portion is further accelerated whereby the electrode is heated. Accordingly, thermorunaway occurs as far as electrode potential is maintained. Such a state has negative resistance, and when current flows over the whole paths, discharge state shifts to arc discharge.
Consequently, it is effective to narrow the electrode interval in plasma generation within a high-pressure range. There exists a lower limit of the electrode interval. In order to allow plasma to exist, the electrode interval at least several times as long as a Debye distance is required. The Debye distance xcex is represented by the following expression.
xcex=(∈0xc2x7xcexaxc2x7Te/q2xc2x7Ne)1/2
where ∈0 is the dielectric constant of vacuum, xcexa is Boltzmann""s constant, q is a charge elementary quantity, Te is temperature of electrons and Ne is density of electric charge.
Since plasma has the electric charge density of about 1015/m3 and the electronic temperature of about 2 eV in an embodiment of the present invention, the Debye distance becomes about 0.3 mm. Therefore, the electrode interval is preferably 1 mm or more.
As mentioned above, although discharge in a pressure range of 1 Torr to 760 Torr is possible, the physical property of plasma is remarkably changed. In the pressure range of about 100 Torr to 760 Torr, as apparent from the above-mentioned shift mechanism to arc discharge, the discharge is liable to be unstable in the usual electrode structure.
Consequently, a heat-resistant dielectric substance is applied onto the surface of the electrode so that a negative resistance is not exhibited in the whole system even though discharge exhibits a negative resistance. Since this dielectric substance has a positive resistance, the whole system provides a positive resistance. In this case, because the dielectric substance is connected in series in the equivalent circuit, it is necessary to make an a.c. electric field develop between the electrodes.
Further, in this range, pressure is high and possibility of collision and re-coupling of ions and electrons in a space is high, whereby plasma is liable to disappear. Therefore, it is necessary to facilitate diffusion of ions and electrons (in particular, diffusion of ions) so as to broaden a plasma region. For that purpose, it is effective to add rare gas having a matasable state, in particular, helium or argon. It is preferable that rare gas is 80% or more of the total gas.
Also, it is preferable to diffuse particles constituting plasma by the aid of a magnetic field. In particular, it is preferable to provide the distribution of a magnetic field so that a magnetic flux diverges from the center of the electrode toward the exterior. If such a magnetic field distribution can be obtained, then electrons drift along the diverged magnetic flux, and positive ions are attracted so that an electric field generated by the electrons is canceled, as a result of which plasma is diffused.
As mentioned above, within the pressure range of about 100 Torr to 760 Torr, it is necessary to insert a dielectric substance onto the surface of the electrode and to add rare gas. However, in the pressure of about 100 Torr or less, the dielectric substance and rare gas are not always necessary. However, the existence of the dielectric substance and rare gas under the pressure of about 100 Torr or less has such an effect that discharge is stabilized. However, there arises problems that the costs are increased and the film forming speed is lowered.
The inventors have observed the physical property of plasma under the pressure of 5 Torr to 750 Torr. Gas used in the experiment is argon, and electrodes between which a dielectric substance is inserted for stabilization of plasma is used. The dielectric substance is formed of a sintered alumina having a thickness of 0.5 mm, and a frequency is 13.56 MHz.
As a typical physical value of plasma, a sustaining voltage necessary for maintaining plasma, the electron temperature (Te), and the electron density (Ne) is measured. The electron temperature and the electron density are measured by using the Langmuir probe method (single probe method), and the sustaining voltage is measured by a terminal voltage of a power source. FIGS. 12 and 14 show measured results.
FIG. 14 shows the electron temperature (Te) and the electron density (Ne). Regarding the electron density, when the probe voltage is applied in a positive voltage direction, an electron saturated current region can be observed. However, there exists a pressure range (60 Torr or more) which cannot be observed in this region. Therefore, since the electron density cannot be calculated in the pressure range, the electron density in 60 Torr or more is not shown. The electron density in 40 Torr or less gradually rises from 1xc3x971014/m3 to 1.7xc3x971014/m3 as pressure rises, and rapidly rises to 8xc3x971014/m3 in a range of 40 Torr to 60 Torr. This shows the fact that arc discharge is locally generated with the boundary of pressure of about 40 Torr, and also shows that plasma in a range of 40 Torr to 60 Torr becomes unstable. However, by using this, a plasma with a considerably high density can be obtained.
FIG. 12 shows the electron temperature (Te) and the sustaining voltage. The sustaining voltage necessary for maintaining plasma is a value representative of facility in dealing with plasma, and is desirably as low as possible. In view of this fact, it is preferable that a minimum value is exhibited in a pressure of 10 Torr to 100 Torr and plasma is used in this pressure range. On the other hand, the characteristic curve of the electron temperature has a minimum value in pressure of 60 Torr and a U-shaped form. The electron temperature in an intermediate pressure range of 15 to 100 Torr is lower than that in a lower pressure and a higher pressure than 10 to 100 Torr, that is, 3 eV or less.
The above-mentioned results are typical and therefore do not exhibit all the results. When the used gas is altered to helium, neon or the like, hydrocarbon gas is added to the used gas, or gas flow rate is changed, then the results are different. For example, the minimum pressure of the electron temperature is changed in a range of 60 Torr to 100 Torr, pressure in which the electron density rapidly increases is changed in a range of 40 to 80 Torr, and pressure in which the sustaining voltage is minimum is changed in a range of 20 Torr to 100 Torr. However, substantially the same results are qualitatively obtained.
As a result of the above-description, it is preferable to lower the sustaining voltage in the intermediate pressure (15 Torr to 100 Torr) in view of facility of using the device, lighting of a power supply and lowering of the costs, and it is preferable to increase electron density in view of an increase in radical density. Furthermore, in the range of the intermediate pressure, because the electron temperature is lowered, it is disadvantageous to generation of radical. However, because plasma potential elevates with respect to an anode which is a grounded potential, bombardment of ions to the anode occurs. This is very convenient to fabricate a hard carbon film located on the anode side. The reason will be described below.
Regarding electrons and ions within plasma, electrons readily move under an electric field having the identical strength because of a difference in mass between electrons and ions, compared with ions. Accordingly, the possibility that electrons reach a vessel becomes higher than that of ions. If the vessel is insulative, then the vessel is negatively charged. If the vessel is conductive, assuming that the vessel contacting with plasma is at the same potential as plasma, current flows in a direction of plasma through the vessel. Since flowing of current is contrary to the condition of charge neutrality, plasma potential is changed in a positive direction with respect to the vessel so that flowing of current is canceled. That is, regardless of the vessel being made of a conductive substance or an insulative substance, plasma is positively charged with respect to the vessel according to a difference of mobility between electrons and ions.
This exhibits that an ion sheath exists on the grounded electrode side. It is certain that the ion sheath also exists on the cathode (voltage supply electrode side). However, usually, because an ion sheath which is naturally generated is sufficiently smaller than the sheath generated by self-bias, it is ignored. An electric field generated by the ion sheath can deal with the ion sheath as an equivalent of a capacitor having an electrical double layer.
Assuming that the speed of electrons is Boltzmann-distributed, the electron density within the ion sheath is reduced exponential-functionally so that a space charge within the ion sheath provides an exponential functional curve. It is appropriate that the boundary between the ion sheath and plasma is defined by a position which has a potential of the degree of Vt=xe2x88x92xcexaxc2x7Te/2 q with respect to bulk potential of plasma. This results from moving the electrons within plasma bulk with energy of the degree of xcexaxc2x7Te/2 q.
As the electron temperature is heightened, a thickness d of the ion sheath is decreased because electrons is infiltrated into the ion sheath, thereby to increase a capacitance C of the electrical double layer. Inversely, as the electron temperature is lowered, the capacitance C of the electrical double layer is reduced.
The charge quantity stored in the ion sheath is proportional to the electrondensity (Ne), that is, the ion density (Ni), and therefore voltage V developed between both ends of the electrical double layer is represented by the following expression.
V=Q/C
=(Ne)2/3xc2x7d/∈0xc2x7S
where d is the thickness of the ion sheath and S is an area of the electrode. That is, as the electron temperature is lowered, the electric field within the ion sheath is more strengthened, and bombardment of ions to the anode is enlarged.
Hitherto, a hard carbon film could not be fabricated on the anode side. On the other hand, in the device of the present invention, a pressure range is from 15 Torr to 100 Torr. Accordingly, the electron temperature is lowered and bombardment of ions is caused even to the anode, whereby the hard carbon film can be formed even on the anode.
In the coating fabricating apparatus in accordance with the present invention, a film which constitutes an object on which a coating has been fabricated is wound on a part of a second cylindrical electrode which is grounded and opposite to a first electrode. The rotation of the second electrode allows the film to pass between the first and second electrodes. A high-frequency voltage is applied to the first electrode whereby a space between the first and second electrodes produces plasma, and raw gas introduced into plasma is activated, as result of which a film is fabricated. A peripheral edge portion of the first electrode is covered with an insulator so that the first and second electrodes and the insulator constitute a substantially closed space. Gas is supplied into the closed space through pores provided in the first electrode. Plasma is shut out within the closed space so that it is difficult to leak plasma to the exterior. An interval between the first and second electrodes is 6 mm or less, pressure within the closed space is 15 Torr to 100 Torr.
As mentioned above, in addition to setting pressure within the closed space to the intermediate pressure, plasma is enclosed in the closed space, thereby preventing discharge in an undesired region. Further, plasma with higher density is generated so that bombardment of ions to the anode is increased.
The undesired region is actually directed to a peripheral portion of the electrode. In the center of the electrode, an electric field has a given or uniform rate of change. However, in the peripheral portion of the electrode, in particular, edge portions of the voltage supply electrode, an electric field strength is enlarged, and discharge concentrates in that region. That is, impedance is lowered with an increase of the plasma density in that region, whereby a large amount of current flows in that region. Therefore, most of electric power is consumed in the peripheral portion of the electrode so that the plasma density is lowered in the center of the electrode. As a result, the electron density is increased, and bombardment of ions in the center of the electrode is lowered.
In the present invention, in order to solve the above-mentioned problem, the peripheral portion of the voltage supply electrode is covered with an insulator in such a manner that plasma is enclosed in the center of the electrode.
Further, in the coating fabricating apparatus in accordance with the present invention, a film which constitutes an object on which a coating has been fabricated is wound on a part of a second cylindrical electrode which is grounded and opposite to a first electrode. The rotation of the second cylindrical electrode allows the film to pass between the first and second electrodes. A high-frequency voltage is applied to the first electrode in such a manner that a space between the first and second electrodes produces plasma. A raw material introduced in plasma is activated to fabricate a coating. The electrodes are constituted so that the strength of an electric field generated on the first and second electrodes is strongest on the surface of the first electrode but weakest on the surface of the second electrode. The shortest interval between the first and second electrodes is 6 mm or less, and pressure between the first and second electrodes is 15 Torr to 100 Torr.
That is, in addition to setting the pressure to the intermediate pressure, the strength of electric field in the periphery of the first electrode (cathode) is heightened in such a manner that the density of plasma is increased in that region. As the shape of the cathode electrode, a knife-shaped or needle-shaped electrode is effective except that edge portions of the plane is used. The ununiform region of the strength of electric field is positively utilized, thereby to obtain plasma with high density.
Further, in the invention, the first electrode to which a high-frequency voltage is applied and the second cylindrical electrode which is grounded are arranged opposite to each other. Plasma is generated between the first and second electrodes by applying the high-frequency voltage thereto. Raw gas introduced in plasma is activated to fabricate a coating. The second electrode is disposed in such a manner that plasma generated between the first and second electrodes is sprayed on the metal surface of the second electrode which is grounded. An object (film) on which a coating has been fabricated is wound on a part of the second electrode. The rotation of the second electrode allows the film to pass through a region on which plasma has been sprayed. The electrode interval is 6 mm or less, and pressure between the first and second electrodes 15 Torr to 100 Torr.
This is a plasma generating apparatus having a structure of a parallel plate or concentric cylindrical electrode, in which plasma with high density can be generated by setting the intermediate pressure. Plasma is positively sprayed onto the substrate by the aid of a gas stream. Because of the intermediate pressure, gas diffusion is delayed in comparison with low pressure, as a result of which there is a case where radical transport rate is determined. However, this problem is solved by spray of plasma.
Further, in the present invention, gas to be supplied within a plasma space consists of mixture gas of hydrogen and gas selected from hydrocarbon, carbon halide and hydro-carbon halide, or mixture gas of its mixture gas and rare gas.
A high-speed film formation can be realized by setting the intermediate pressure, however, there arises a problem such as adhesion of the film onto the cathode. This can be solved by adding carbon halide thereto.
In the present invention, bombardment of ions onto the anode side is effected thereby being capable of fabricating a hard carbon film even on the anode side. However, since self-bias applied on the cathode side is larger than that on the anode side, bombardment of ions on the cathode side is stronger than that on the anode side. In this embodiment, using this phenomenon, halogen based gas having etching action is added to raw gas whereby no film formation but etching is conducted on the cathode side.
Carbon halide, for example, carbon tetrafluoride is known as etching gas. Carbon tetrafluoride has only etching action, and in dicarbon hexafluoride, tricarbon octafluoride or the like, etching or film formation is performed according to the strength of self-bias. That is, in the case where self-bias is strong and bombardment of ions is also strong, etching is performed. Inversely, in the case where self-bias is weak and bombardment of ions is also weak, the film formation is performed. In the present invention, bombardment is stronger on the cathode side onto which it is not desired to form a film, which is very convenient to the invention.
Accordingly, film fabrication on the cathode side can be restrained and occurrence of flakes can be restrained. Further, the maintenance duration for the apparatus can be extended, thereby greatly contributing to an improvement in through-put and a reduction in the costs. In the case of a super LSI process or the like, it causes contamination, however, in fabricating a carbon film as in the present invention, there does not arise a problem such as contamination. When the film which constitutes an object is electrically conductive, a film can be disposed not on the cathode side but only on the anode side. The present invention is effective even in such a case.